A non-aqueous secondary battery represented by a lithium ion secondary battery which uses, as a positive electrode, lithium-containing transition metal oxides as represented by lithium cobaltate and uses, as a negative electrode, carbon material which is capable of doping and dedoping lithium is characterized by having a high energy density. By this characteristic, a non-aqueous secondary battery is important as a battery for portable electronic appliances represented by a cellular phone. Along with the fast popularization of these portable electronic appliances, demand therefor is ever-increasing.
Large numbers of vehicles which are conscious of environmental responsiveness, such as hybrid vehicles, are being developed. As a power source mounted on a vehicle, a lithium ion secondary battery having a high energy density is attracting a great deal of attentions.
Most lithium ion secondary batteries are composed of a layered body of a positive electrode, a separator containing electrolyte and a negative electrode. A principal function of the separator is to prevent short circuit between a positive electrode and a negative electrode, and examples of required properties of the separator include permeability of lithium ion, strength and durability.
At present, as a film suitable for a separator for lithium ion secondary battery, a large number of varieties of polyolefin microporous membranes are proposed. A polyolefin microporous membrane satisfies the above-mentioned required properties and has, as a safety function at high temperature, a so-called shutdown function, which is a thermal runway preventing function by shutting down the current by blocking holes due to a high temperature. Therefore, a polyolefin microporous membrane is widely used for a separator for lithium ion secondary battery.
However, there are cases where the temperature inside the battery exceeds the melting point of polyethylene constituting a microporous membrane, even when the shutdown function works and the holes of polyethylene microporous membranes are blocked whereby the electric current is temporarily shutdown. When the limit of the heat resistance of the polyolefin microporous membrane is exceeded, the microporous membrane per se melts, and the shutdown function is lost. As a result, a short circuit between electrodes triggers heat runaway of the battery. At this point, a breaking in a device in which the lithium ion battery is installed or an accident due to ignition may occur. For this reason, in order to ensure further safety, a separator which can maintain the shutdown function even at a high temperature is demanded.
Therefore, in Patent Document 1, proposed is a separator for non-aqueous secondary battery in which the surface of a polyethylene microporous membrane is covered with a heat-resistant porous layer composed of heat resistant polymer such as fully aromatic polyamides. In Patent Document 2, disclosed is a configuration in which inorganic particulates such as alumina are contained in a heat-resistant porous layer, to thereby improve a heat resistance as well as a shutdown function. In Patent Document 3, disclosed is a configuration in which metal hydroxide particulates such as aluminium hydroxides are contained in a heat-resistant porous layer, to thereby improve a flame resistance as well as a shutdown function and a heat resistance. In these configurations, excellent effects can be expected from the viewpoint of the safety of the batteries on this point that the shutdown function and the heat resistance are gone together.